Arm backend: Fix int64 argmax delegation rejection

[zingo]

TOSA 1.0 ARGMAX returns int32 indices, while aten.argmax returns int64. If the graph consumes the raw aten.argmax result on CPU fallback, delegating ARGMAX changes the dtype and can fail with an Int-vs-Long mismatch.

Keep delegation enabled only when every user immediately casts the argmax result to int32:

aten.argmax
-> _to_dim_order_copy(dtype=int32)
-> users

Otherwise reject the node during support checking so the PyTorch-compatible int64 result is preserved.

Fixes the flow test:

test_argmax_dim[arm_tosa_fp]

cc @digantdesai @freddan80 @per @oscarandersson8218 @mansnils @Sebastian-Larsson @robell @rascani

[pytorch-bot]

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❌ 3 New Failures, 9 Pending, 1 Unrelated Failure

As of commit 6f55027 with merge base b1ef9a5 ():

NEW FAILURES - The following jobs have failed:

• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 8b2f882fc7442246fefda957b1cee8af2283e097c983373c6d70065a558f6c0b /exec failed with exit code 1
• pull / unittest-editable / linux / linux-job (gh)
  RuntimeError: Command docker exec -t e1346b8c9c599bff1fc5850f667066fe9438870fc6ce3d5b21feaf0968026135 /exec failed with exit code 1
• trunk / unittest-release / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 42c678a1b7209dacd669d2a19afe0948d34b94276e841cc7be81cb46b9cb48d0 /exec failed with exit code 1

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

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